The present invention concerns a mobile telephony method and system. A system of the kind concerned is referred to as a xe2x80x9cGlobal System for Mobile communicationsxe2x80x9d (GSM). To describe it in a somewhat simplified manner, it includes mobile stations carried by respective users of the mobile stations, who are subscribers of the system. The mobile stations provide radio links with Mobile service Switching Centers (MSC). The MSCs have respective coverage areas. Each mobile station enables its user to be one party to a call linking it to at least one other party via a center visited by the mobile station. The center is the MSC in whose coverage area the mobile station is temporarily located. Beyond the center, the call in question travels through the remainder of the system and possibly, if the other party is not a subscriber of the system, via the Public Switched Telephone Network (PSTN).
To enable call set-up and billing, each MSC stores the identity of each mobile station visiting the center and a Home Location Register (HLR) stores data concerning all the mobile stations of the system. The data in the HLR include the locations of the mobile stations, i.e. the identities of the MSC visited by the mobile stations.
Many messages are sent to the HLR to ask it for information needed to set up calls passing through the system because at least one of the parties to be linked by such calls is a subscriber of the system. This is why the data in the HLR is individually updated each time a mobile station moves from one area to another and after each change of status of a user. The user""s status defines conditions under which the user has access to the system and is billed. It is part of the data concerning the user""s mobile station. The data is collectively backed up in another register. These operations are carried out periodically so that the back-up register does not contain locations of mobile stations which have moved since the last such operation. Some modifications, such as those concerning the status of some categories of users, are done in the back-up register first.
Following any such modification in the back-up register, or following other incidents that can affect the validity of the data in the HLR, the data is collectively updated in an operation which is referred to as a xe2x80x9creset,xe2x80x9d that is based on data from the back-up register. The reset is reported to the MSC. On the first contact of a mobile station with the MSC it is visiting after a reset, the center sends the HLR a message to harmonize the data concerning the mobile station in the MSC and in the HLR. The location of the mobile station is updated in the HLR on the basis of the identity of the center sending the message and the data defining the status of the user of the mobile station is updated in the center from data contained in the HLR.
Prior art systems of the above kind have the disadvantage that, when a system of this kind is requested to set up a call, call set-up is sometimes impeded by unavailability of the system, in other words the call request that requests call set-up is not completed even though the called party is available.
An aim of the present invention is to provide a simple and low-cost way to limit such unavailability of the system. With this aim in view, the invention consists in a method which conventionally includes operations performed by a HLR containing respective data on a plurality of mobile stations. Each such operation consists in exploiting a message triggering the operation. To be able to select some of the messages in order to be able to exploit them in an overload situation, the HLR assigns each message a priority level chosen from a scale of such levels.
Some of the operations effected by the HLR are collective updates of mobile station data. Each such update constitutes a reset and is reported to a plurality of MSCs each containing the data of the mobile stations located in the area of the center.
Other operations are individual updates. Each such update consists in exploiting a message transmitted by a MSC and updates in the HLR the data of a mobile station constituting a station to which the message relates.
Some of the individual updates are visit updates. A message requesting a visit update is transmitted by a MSC to exploit a visit notification reporting the arrival of the mobile station to which the message relates in the area of the center, the message constituting a visit request.
Other individual updates are referred to as updates after a reset. A message requesting an update after a reset is transmitted by a MSC to exploit a contact between the center and the mobile station to which the message relates. It constitutes a request after a reset. It is transmitted when three conditions are combined:
the mobile station is in the area of the center,
the contact is after a preceding message transmitted by the center relating to the mobile station and requesting a visit update, and
the contact is a first contact between the mobile station and the center since a reset was reported to the center.
In the method of the invention the MSCs introduce a difference between visit update requests and update after a reset requests detectable by the HLR and by means of which the HLR assigns update after a reset requests a lower priority level than visit update requests.
In the context of the present invention, it has been found that many instances of unavailability in prior art systems were related to the bit rate of messages sent to the HLR by the MSCs, the bit rate sometimes exceeding the HLR""s capacity to exploit such messages. It has been realized that abstaining from or delaying the exploitation of such messages would lead directly to only a few instances of unavailability of the system if such abstention were in practice limited to messages received by the HLR after a reset and at the same time as other messages needed to set up a call currently requested or requested afterwards. More particularly, it has been realized that there are then many fewer instances of unavailability than occur on abstaining from responding to the other messages in prior art systems. The present invention therefore avoids the additional cost that would be generated by increasing the HLR""s capacity for exploiting the messages it receives, at least when the increase in capacity would be sufficient to eliminate instances of unavailability linked to messages sent after a reset.